Upright bars allow water to pass through while obstructing creature movement. The water can reach dangerously low levels during draining, though, which can result in premature drowning. I've used the straining technique on large fish before with great success; if only it weren't for the hippos... the hippos... oh god, the hippos...
And while I don't have a chasm, I do have an aquifer below sea level, so I may be able to generate some movement in my ocean... but again, I worry that straining might cause premature deaths. Still, it's worth a try if I can get the merpeople back... I may have inadvertently killed my entire current population of merpeople in a small construction mishap...
Anyhow, thanks for all the contributions! My plan now is to create "roach motel" style traps using measured lava-dropping for obsidian ring-walls, using those as a base for my screw pumps, and draining the center to place the cage traps before channeling individual sections of the obsidian wall out to place my floodgate entrances individually. I figure the backwash issue will slow things down much less with only single inlets flowing at any given time. But who knows? Maybe the result will be 3X the water pressure on a normal square in open ocean pump-ring and I'll have construction workers pushed back by the force of the flow!
Here are the rough blueprints:
The floor hatches being supported by up stairs, and made of bauxite, with bauxite mechanisms linking them to levers. I'd already produced enough for the internal pillar component, which I intended to use in order to dig a staircase to the seabed, before even realizing that the merpeople presented such a potentially lucrative project... but if I want to do this, I may need to order more bauxite. My stock is getting fairly low, and my lava-works will have to go further into the ocean than I'd intended... anyhow, this should produce:
Which will be pumped out and carved into some other stuff, and then stuff. Not this weekend, though! I'm off now, even before I can finish posting my plans, and shall be very busy for a bit... but those frolicking finned money sacks shall be mine...